The T-operator can be obtained analytically using Mie theory for simple objects (e.g., particles with spherical symmetry) and numerically by solving Maxwell's equations otherwise [1]}. Interestingly, for assemblies of particles, one can define a T-operator of the whole system, which can be expressed in terms of the T-operators of the individual particles. The computation can be achieved by solving the multiple-scattering problem using, for instance, so-called the T-matrix method [2]}.
